Select Myrobalan tree ‘RI-1’

ABSTRACT

A new and distinct variety of Myrobalan tree ( Prunus cerasifera ). Its novelty consists of the following unique combination of features that are desirable in a new interstock when growing cherries: 
     1. Decreasing the height of the cherry tree for more economical harvesting. 
     2. Providing earlier production of fuit when growing cherries with the new interstock variety on Citation Rootstock. 
     3. Increasing the number of rootstocks available for cherries. 
     4. Improving the opportunity to match soil type with the desired rootstock by using an interstock.

Latin name: Prunus cerasifera.

Variety denomination: RI-1.

BACKGROUND OF THE VARIEETY

In the field of plant genetics, we conduct an extensive and continuingplant-breeding program including the organization and asexualreproduction of orchard trees, and of which plums, peaches, nectarines,apricots, cherries, interspecifics and rootstocks are exemplary. It wasagainst this background of our activities that the present variety ofrootstock-interstock was originated and asexually reproduced by us in anexperimental orchard located near Modesto, Stanislaus County, Calif.

PRIOR VARIETIES

Among the existing varieties of rootstocks which are known to us, andmentioned herein, are ‘Nemaguard’ Rootstock (non-patented), ‘AtlasRootstock’ (U.S. Plant Pat. No. 8,913), ‘Viking ’ Rootstock (U.S. PlantPat. No. 8,912), Citation™ Rootstock (U.S. Plant Pat. No. 5,112),‘Mazzard’ Rootstock (non-patented), and ‘Mahaleb’ Rootstock(non-patented).

ORIGIN OF THE VARIETY

It is well known that sweet cherry trees are compatible with a verylimited number of rootstocks for commercial production. For many years,the primary rootstocks available for the production of commercial cherrytrees have been ‘Mahaleb’ (non-patented) and ‘Mazzard’ (non-patented).Both of these varieties are undesirably tall, and have a long juveninityperiod, delaying production of fruit for five to seven years.Additionally, ‘Mahaleb’ is susceptible to phytophthora root rot and overcropping, which results in small fruit size. ‘Mazzard’ is a strongertree with lighter production, but is even taller than ‘Mahaleb’.

Due to the limitations of the rootstocks presently available forcommercial cherry trees, we attempted to identify a new interstock thatwould be compatible with a variety of commercially available rootstocks,as well as with sweet cherry varieties.

The present new Myrobalan tree (Prunus cerasifera) was originated by usin our experimental orchard located near Modesto, Calif. as a selectionfrom seed of open pollinated populations of Myrobalan rootstocks. Theexact parentage of the new variety is unknown. We planted and maintaineda large group of these open pollinated Myrobalan seedlings growing ontheir own root system in our experimental orchard. We budded and graftedthese seedlings to various rootstocks and various varieties of cherries(Prunus avium), searching for an interstock between the various species.We experienced some success with several combinations, but one seedling,now known as ‘RI-1’, showed particular promise as an interstock. Thatseedling was tested by budding and grafting and found to be compatiblewith cherries and several rootstocks, as was thus selected for asexualpropagation and commercialization as an interstock for use with cherryscion wood.

‘RI-1’ is distinguishable from other cultivars of the species by itscompatibility with cherry trees. Specifically, the ‘RI-1’ cultivar, whenused as an interstock, exhibits significantly less overgrowth at theunion of the interstock and scion, as compared with other knownMyrobalan cultivars, and exhibits no overgrowth at the union of therootstock and interstock. The new interstock is graft compatible with avariety of prunus rootstocks, which allows growers to select the mostdesirable rootstock for a particular type of soil.

ASEXUAL REPRODUCTION OF THE VARIETY

Asexual reproduction of the new and distinct variety of Myrobalan treewas by budding to Citation™ Rootstock (U.S. Plant Pat. No. 5,112) asperformed by us in our experimental orchard located near Modesto,Calif., and shows that reproductions run true to the original tree andall characteristics of the tree and its fruit are established andtransmitted through succeeding asexual propagations.

SUMMARY OF THE NEW VARIETY

The new variety of Myrobalan tree (Prunus cerasifera) is of medium size,moderately vigorous, upright in growth and graft compatible with amoderate range of prunus species, which makes it available to be used asan interstock between cherries (Prunus avium) and the interspecificrootstock Citation™ (U.S. Plant Pat. No. 5,112) (Prunus salicina×Prunuspersica). Budding and grafting tests show compatibility with thefollowing rootstocks and scions: ‘Nemaguard’ Rootstock (non-patented),Citation™ Rootstock (U.S. Plant Pat. No. 5,112), ‘Viking’ Rootstock(U.S. Plant Pat. No. 8,912), ‘Atlas’ Rootstock (U.S. Plant Pat. No.8,913), ‘Craigs Crimson’ Cherry (U.S. Plant Pat. No. 7,320),‘Earlisweet’ Cherry (U.S. Plant Pat. No. 9,783), ‘Royal Rainier’ Cherry(U.S. Plant Pat. No. 10,790) and the following non-patented cherries:‘Bing’, ‘Van’, ‘Rainier’, ‘Black Tartarian’, ‘Stella’, ‘Lapins’ and 31seedling selections of cherries in the advanced testing stage. We expectthat similar compatibility will be exhibited with other cherryvarieties.

Cherries growing on the new Myrobalan interstock grafted to Citation™rootstock (“Citation-Myrobalan-cherry composite tree”) have beenobserved over a fourteen (14) year period. We have observed a moderatevariability in the amount of dwarfing effect within cherry varietieswhen using the same length of interstock. Decreasing the length ofinterstock reduces the size of the tree in all cherry varieties, somevarieties with a greater degree of dwarfing than other varieties. Theinterstock should be at least about four inches (approximately 10 cm) inlength, and should be no more than about 18 inches (approximately 46 cm)in length. A 6 inch (approximately 15 cm) interstock is preferable. Forexample, ‘Earlisweet’ Cherry can be reduced to approximately one-thirdof normal tree size and ‘Rainier’ Cherry reduced to approximatelyone-half normal tree size when budded to a 6-inch segment of the newMyrobalan interstock growing on Citation™ Rootstock. The combination ofthe new Myrobalan interstock with Citation™ Rootstock has increasedprecociousness with cherries we have tested to start fruit production inthe third year. Crop load is relatively heavy.

The Myrobalan interstock union on the above rootstocks, includingCitation™, is relatively smooth, while the bud union on the cherry scionis more prone to be slightly over-grown. The amount of over growthvaries within cherry varieties. This same type of overgrowth can befound when growing cherries without an interstock on standard cherryrootstocks such as ‘Mahaleb’. We have had no tree loss with the newMyrobalan interstock from breaking at bud union of either cherry orrootstock.

The Citation-Myrobalan-cherry composite tree has been grown in our testorchards and shows promise as commercial fruit tree. Based on ourobservations of the Citation-Myrobalan-cherry composite tree, we expectthat related composite trees will have similar qualities. Such compositetrees could include any of the rootstocks named above, or any prunus orinterspecific rootstocks that are compatible with the new Myrobalaninterstock. The composite trees could further include any of the sweetcherry varieties named above, or any sweet cherry varieties that arecompatible with the new Myrobalan interstock.

PHOTOGRAPH OF THE VARIETY

The accompanying color photographic illustration shows typical specimensof the foliage and fruit of the present new Myrobalan variety. Theillustration shows the upper and lower surface of the leaves, anexterior and sectional view of a fruit divided in its suture plane toshow flesh color, pit cavity and the stone remaining in place. Thephotographic illustration was taken shortly after being picked and thecolors are as true as is reasonably possible in a color representationof this type.

DESCRIPTION OF THE VARIETY

The following detailed botanical description of the new Myrobalan tree,its flowers, foliage and fruit, is based on an exemplary tree, budded onCitation™ Interspecific Rootstock (U.S. Plant Pat. No. 5,112), grown onHandford sandy loam soil with Storie Index rating 95, in USDA HardinessZone 9, near Modesto, Calif., with standard commercial fruit growingpractices, such as pruning, thinning, spraying, irrigation andfertilization. At the time the description was recorded, the tree was 7years old. Color terminology (except those in common terms) is inaccordance with Reinhold Color Atlas by A. Kornerup and J. H. Wanscher.

Tree:

Size.—Medium to large. Tree pruned severely each winter to induce propernew growth for budwood.

Vigor.—Moderately vigorous. Tree growth of 5 to 6 feet (1.5 to 1.8 m) inheight and 3 to 4 feet (0.9 to 1.2 m) in width the first growing season.

Growth.—Upright. Each dormant season tree is cut back to 4 to 6 feet(0.9 to 1.8 m) in height to induce rapid new growth for spring budwood.

Branching habit.—Low apical dominance allows numerous small branches tocreate a feathered condition throughout the tree.

Productivity.—Tree very productive, fruit has no commercial value.

Bearer.—Regular.

Density.—Dense. Numerous lateral branching increases density. Tree iscut back to 5 to 6 feet (1.5 to 1.8 m) in dormant season to promoterapid growth for desirable budwood.

Hardiness.—Hardy in all stone fruit growing areas in California.

Trunk:

Size.—Medium to large. Circumference 19 inches (48 cm) measured 9 inches(23 cm) above ground on a 7 year old tree. Varies slightly with soiltype, climatic conditions and cultural practices.

Surface texture.—Medium shaggy. Roughness increases with age of tree.

Color.—Brown to soot brown (5-F-4) to (5-F-5).

Branches:

Size.—Medium. Average circumference 9 inches (23 cm), measured 58 inches(147 cm) above ground. Crotch angle approximately 30 degrees.

Surface texture.—New growth smooth. Mature growth medium rough.

Lenticels.—Medium to large. Average length — ⅛″ (3.2 mm); Average width— {fraction (1/16)}″ (1.6 mm); Color varies from grayish orange togolden yellow (5-B-6) to (5-B-8); Numerous — 31 in 4 square inch (26 sq.cm) surface.

Color.—New growth varies from olive brown (4-D-6) on under side toviolet brown on upper side exposed to sun; Mature growth varies fromlight brown to hair brown (5-D-5) to (5-E-5).

Leaves:

Size.—Small; Average length — 1¾ inches (4.4 cm); Average width — ⅞ inch(2.2 cm).

Form.—Ovate.

Apex.—Acuminate.

Base.—Obtuse.

Margin.—Double serrate.

Thickness.—Medium.

Surface texture.—Upper surface relatively smooth, only slightly indentedon upper surface over leaf vein; Lower surface relatively smooth, midribwith pinnate veining create small ridges.

Petiole.—Color — grayish yellow (2-B-4); Average length — {fraction(19/64)} inch (7.5 mm); Average width — {fraction (3/64)} inch (1.2 mm).

Color.—Upper surface — green to dark green (27-F-6) to (27-F-8). Lowersurface — light green to jade green (27-E-5) to (27-E-7).

Midvein.—Pronounced; Midvein same width as petiole for approximately ½of leaf; Color — grayish yellow (2-B-4); Venation — pinnate.

Glands.—Eglandular.

Flowers buds:

Quantity per spur.—Average quantity 6, varies from 1 to 12.

Size.—Small. Average length — {fraction (21/64)} inch (8.3 mm). Averagediameter — {fraction (13/64)} inch (5.2 mm).

Hardiness.—Hardy in all stone fruit growing areas of California. Grownin Zone 9.

Form.—Plump, free; Varies from conic to elongated as bud developmentprogresses.

Color.—White (1-A-1).

Pedicel.—Average length — {fraction (29/64)} inch (11.5 mm). Averagewidth — {fraction (3/64)} inch (1.2 mm). Color — grayish green (1-D-5).

Flowers:

Development.—Perigynous, complete, single pistol, multiple stamens, fivepetals and sepals alternately arranged.

Size.—Small. Average height — {fraction (21/64)} inch (8.3 mm). Averagediameter — {fraction (55/64)} inch (21.8 mm).

Color.—White (1-A-1).

Pistil.—Usually one. Average length — {fraction (5/16)} inch (7.9 mm).Color — white (1-A-1).

Stamens.—28 to 31 per flower. Average filament length — ¼ inch (6.4 mm).Color — white (1-A-1). Average stamen length — {fraction (17/64)} inch(6.7 mm), same height as pistils in mature flower. Anther color — lightyellow to yellow (4-A-5) to (4-A-6).

Petals.—Quantity — five, alternately arranged to sepals. Shape —obovate, narrows at point of attachment. Size — medium. Average length —{fraction (23/64)} inch (9.1 mm). Average width — {fraction (19/64)}inch (7.5 mm). Surface edges vary from smooth to slightly scalloped.Color — white (1-A-1).

Sepals.—Quantity — five, alternately arranged to petals. Glabrous. Shape— ovate, somewhat triangular, tip rounded. Average length — {fraction(5/32)} inch (4.0 mm). Average width at base — {fraction (3/32)} inch(2.4 mm). Color — brownish red (10-D-7) on lower center to grayish green(1-D-5) along outer edges.

Pollen.—Abundant. Unknown as to self-fertility. Unimportant as to valueof interstock.

Fragrance.—Very slight.

Blooming period.—Date of first bloom Feb. 27, 2000. Date of last bloomMar. 7, 2000. Varies slightly with climatic conditions.

Flower stem:

Size.—Medium. Average length — {fraction (15/64)} inches (6.0 mm).Average width — {fraction (3/64)} inch (1.2 mm).

Color.—Grayish yellow (2-C-4).

Fruit:

Maturity when described.—Firm ripe, no commercial value.

Date of fruit ripening.—Jun. 22, 1999. Fruit drops on ground, nocommercial value.

Form.—Globose, slightly elongated.

Size.—Small. Average diameter axially {fraction (15/16)} to 1 inch (23.8to 25.4 mm). Average transversely in suture plane ¾ to {fraction(15/16)} inch (19.0 to 23.8 mm). Average weight 7.9 grams. Averageweight varies with quantity of fruit per tree, fertility of soil andclimatic conditions.

Suture.—Smooth. Extends from base to apex.

Ventral surface.—Rounded, smooth.

Apex.—Slight, rounded.

Base.—Flat, varies from flat to slightly retuse.

Cavity.—Rounded. Very slightly elongated in suture plane. Average depth— {fraction (1/16)} inch (1.6 mm). Average breadth — ⅛ inch (3.2 mm).

Stem:

Size.—Small. Average length — ⅜ inch (9.5 mm). Average width — {fraction(1/16)} inch (1.6 mm).

Color.—Grayish green to spring green (30-C-6) to (30-C-7).

Flesh:

Ripens.—Even.

Texture.—Firm.

Fibers.—None.

Aroma.—Slight.

Eating quality.—Poor, no commercial value.

Flavor.—Acid.

Juice.—Moderate.

Color.—Pale yellow to butter yellow (4-A-4) to (4-A-5). Pit cavity color— golden yellow (4-B-5).

Skin:

Color.—Yellow to amber yellow (4-A-6) to (4-B-6).

Thickness.—Medium.

Texture.—Medium.

Bloom.—Moderate.

Tendency to crack.—Slight in wet weather.

Tenacity.—Tenacious to flesh.

Stone:

Type.—Clingstone.

Size.—Small. Average length {fraction (35/64)} inch (13.9 mm). Averagewidth {fraction (25/64)} inch (9.9 mm). Average thickness — {fraction(17/64)} inch (6.8 mm).

Form.—Ovoid.

Base.—Usually round. Varies from rounded to straight.

Apex.—Acuminate. Length — short, {fraction (1/16)} inch (1.6 mm).

Surface.—Pitted throughout. Pits small and relatively uniform. One veryshallow groove on each side of suture.

Sides.—Equal.

Tendency to split.—None.

Color.—Light brown to oak brown (5-C-5) to (5-D-5) when dry.

Use: Rootstock interstock for tree size control with cherries (Prunusavium).

Disease resistance/susceptibility: No specific testing for relativeplant/fruit disease resistance/susceptibility has been designed. Underclose observation during planting, growing, and harvesting of fruit,under normal cultural and growing conditions near Modesto, Calif., noparticular plant/fruit disease resistance or susceptibility has beenobserved. Any variety or selection observed during indexing of plantcharacteristics, with abnormal fungus, bacterial, virus, or insectsusceptibility is destroyed and eliminated from our breeding program.

The present new variety of Myrobalan tree, its flowers, foliage andfruit herein described may vary in slight detail due to climate, soilconditions and cultural practices under which the variety may be grown.The present description is that of the variety grown under theecological conditions prevailing near Modesto, Calif.

We claim:
 1. A new and distinct variety of Myrobalan (Prunus cerasifera) tree, substantially as illustrated and described, characterized by being medium to large in size, moderately vigorous, upright growth and having graft compatibility with numerous rootstocks and also with various cherry trees, giving this Myrobalan tree the potential to be used as an interstock for growing cherries on various non-compatible rootstocks; the new variety is further characterized by having the ability to reduce the size of the cherry tree by shortening the length of the interstock between the cherry scion and the rootstock. 